These are replaced when we package the final version for release. We have a multitude of scripts and programs that optimize & minify code for better performance - and they also set these placeholders. These tools include a lot of internals though, so we do not share them publicly.
The REPLACE_BY_BUILD
exists to make it impossible for the browser to cache, when we release a new version. We´ve tried many ways to invalidate browser caches, but this was the most reliable by far.